Hey, folks: The image that I posted here a few days ago, purporting to be from the Australian Radiation Services, appears to be a hoax and so I've removed it. The ARS says it did not produce such a map, and I regret jumping to the gun on this.
However, the BEST scenario right now is a series of controlled releases of radioactive steam over the next weeks or months. The article linked to also notes that radioactive particles have been detected up to sixty miles away from the damaged reactor SO FAR.
As for whether radioactive contamination could reach the US, consider things like global travel and global trade. Consider also that California's two nuclear plants are built near the San Andreas Fault, that there is a 88% chance of a magnitude 6.7 earthquake occurring in California in the next thirty years with a 46% chance of 7.5 or greater -- the maximum intensity that the state's nuclear plants are built to handle.
In all the talk about energy, most of the conversation still revolves around oil, coal, and wind -- three things that can only be harnessed by major corporations and sold. The sun and wind and geothermal energy are there for everyone who can spring for the five figures for the equipment, which makes them unlikely to receive much Federal funding. After all, if it doesn't benefit campaign contributors, our government isn't going to do a thing with it.